Determination of different gonadotropin synthesis mechanisms in trophoblast and choriocarcinoma cells

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Human chorionic gonadotropin (hCG) is a glycoprotein hormone consisting of noncovalently bonded subunits, alpha and beta. Both subunits contain two Asn(N)-linked sugar chains, in addition, the beta-subunit contains four Ser(O)-linked sugar chains. It has been suggested that altered glycosylation is induced in both alpha- and beta-subunits of hCG produced by choriocarcinoma. The objective of this study was to determine the nature of the structural differences of hormones synthesized in the cells and secreted as well as the uncombined subunits in normal trophoblasts and a choriocarcinoma cell line (BeWo).The results obtained from this study are as follows:1. Intracellular forms of hCG subunits in normal trophoblast cells were analyzed by immunobinding method. It was revealed that hCG subunits accumulate in trophoblast cells as predominant intermediates (immature forms) containing high-mannose sugar chains. On the other hand, Ser-linked sugar chain was not found in the immature beta-subunit.2. The immature hCG subunits were purified, separately, and their physicochemical properties were determined. N-terminal amino acid sequence of each immature subunit was identical to respective mature form. The purified immature subunits were recombined each other and reconstituted an immature hCG. The apparent molecular weights of the immature hCG found in trophoblast cells as well as hCG reconstituted with the purified immature subunits were larger than those expected. It was suggested that the immature hCG is present in trophoblast cells as an oligomer.3. The structural characteristics of hCG and its subunits in choriocarcinoma, BeWo cells, were compared to those of normal trophoblast cells. It was found that both protein and sugar chain structures of hCG subunits of BeWo cells were differed from those of trophoblast cells.